A prior art locking valve for use primarly with hydraulic cylinders is illustrated in a catalog titled Hydraulic and Pneumatic Check and Relief Check Valves, published by Kepner Products Company, Villa Park, Ill.. Those prior art locking valves are pilot operated such that release of the locking valves requires application of fluid pressure to a pilot port of the locking valve. Such locking valves do not permit release of the locking valve in the absence of a fluid pressure source.
Attention is also directed to the Kepner U.S. Pat. No. 2,959,188, issued Nov. 8, 1960; the Kepner U.S. Pat. No. 3,335,750, issued Aug. 15, 1967; Kaetterhenry U.S. Pat. No. 4,018,136, issued Apr. 19, 1977; and the Panis U.S. Pat. No. 3,975,987, issued Aug. 24, 1976. Attention is also directed to the Gerulis U.S. Pat. No. 4,192,338, issued Mar. 11, 1980; the Bobnar U.S. Pat. No. 4,172,582, issued Oct. 30, 1979; the Bitonti U.S. Pat. No. 3,980,336, issued Sept. 14, 1976; the Martin U.S. Pat. No. 3,817,154, issued June 18, 1975; the Parrett et al U.S. Pat. No. 3,792,715, issued Feb. 19, 1974; the Reith U.S. Pat. No. 4,531,449, issued July 30, 1985; and the Bingel U.S. Pat. No. 3,229,721 issued Jan. 18, 1966.